Magnetic Activated Carbon from ZnCl2 and FeCl3 Coactivation of Lotus Seedpod: One-Pot Preparation, Characterization, and Catalytic Activity towards Robust Degradation of Acid Orange 10

Lotus seedpods (LSPs) are an abundant and underutilized agricultural residue discarded from lotus seed production. In this study, ZnCl2 and FeCl3 coactivation of LSP for one-pot preparation of magnetic activated carbon (MAC) was explored for the first time. X-ray diffraction (XRD) results showed that Fe3O4, Fe0, and ZnO crystals were formed in the LSP-derived carbon matrix. Notably, transmission electron microscopy (TEM) images showed that the shapes of these components consisted of not only nanoparticles but also nanowires. Fe and Zn contents in MAC determined by atomic absorption spectroscopy (AAS) were 6.89 and 3.94 wt%, respectively. Moreover, SBET and Vtotal of MAC prepared by coactivation with ZnCl2 and FeCl3 were 1080 m2/g and 0.51 cm3/g, which were much higher than those prepared by single activation with FeCl3 (274 m2/g and 0.14 cm3/g) or ZnCl2 (369 m2/g and 0.21 cm3/g). MAC was subsequently applied as an oxidation catalyst for Fenton-like degradation of acid orange 10 (AO10). As a result, 0.20 g/L MAC could partially remove AO10 (100 ppm) with an adsorption capacity of 78.4 mg/g at pH 3.0. When 350 ppm H2O2 was further added, AO10 was decolorized rapidly, nearly complete within 30 min, and 66% of the COD was removed in 120 min. The potent catalytic performance of MAC might come from the synergistic effect of Fe0 and Fe3O4 nanocrystals in the porous carbon support. MAC also demonstrated effective stability and reusability after five consecutive cycles, when total AO10 removal at 20 min of H2O2 addition slightly decreased from 93.9 ± 0.9% to 86.3 ± 0.8% and minimal iron leaching of 1.14 to 1.19 mg/L was detected. Interestingly, the MAC catalyst with a saturation magnetization of 3.6 emu/g was easily separated from the treated mixture for the next cycle. Overall, these findings demonstrate that magnetic activated carbon prepared from ZnCl2 and FeCl3 coactivation of lotus seedpod waste can be a low-cost catalyst for rapid degradation of acid orange 10.


Introduction
Today, agricultural activities generate enormous amounts of solid wastes all over the world. Tese agricultural wastes are commonly disposed of by burning them in the felds. Tis activity can cause a variety of ecological and environmental problems [1]. Hence, numerous studies on the valorization of agricultural residues have been done in light of diferent economic, energy, and environmental concerns [2,3]. Typically, agricultural wastes consist of lignocellulosic biomass, which includes cellulose, hemicellulose, and lignin [4]. Due to their carbon resources, these wastes can be employed in the production of carbon-based materials [5,6].
Biochar (BC) is a carbon-rich material prepared from the pyrolysis of diferent biomass resources in oxygen-free environments [7][8][9][10][11]. Despite the vast variety of carbon-based materials, BC is an inexpensive, readily available, and convertible material [12,13]. Moreover, it possesses advantageous physicochemical features, porous structures, and varied functional groups [14]. Tus, BC is widely utilized for gas storage and separation, soil treatment, wastewater treatment, electrodes, and energy storage [15][16][17][18]. Regardless of this, it is challenging to separate BC from its suspension [19,20]. Traditional separation techniques are often expensive or insufcient, thereby severely limiting the application of BC [21]. Consequently, introducing magnetic components into BC can overcome this disadvantage. Diferent magnetic components such as Fe 0 , Fe 2 O 3 , Fe 3 O 4 , and MnFe 2 O 4 particles could be dispersed on BC, resulting in a new material known as magnetic biochar (MBC).
To synthesize MBC from biomass, magnetic precursors are commonly loaded onto carbon surfaces [22]. Tis old approach, however, is not only complex, but it also closes existing pores in carbon supports [23]. In recent years, a growing number of publications focusing on producing MBC using one-pot pyrolysis of magnetic precursor-loaded biomass have been developed [24,25]. It involves directly dispersing magnetic precursors like FeCl 3 into biomass resources and then pyrolyzing the obtained mixtures to yield MBC [26,27]. According to Bedia et al. [28], biomass activated with FeCl 3 produces MBC with well-dispersed ironbased nanoparticles and well-developed porosity. However, compared with other well-known activating agents, FeCl 3 has limited activation efciency. Porous systems of MBC grow slightly. For instance, specifc surface areas (S BET ) of MBC are obtained from municipal sludge (FeCl 3 /N 2 ): 38 m 2 / g [29]; spent cofee grounds (FeCl 3 /N 2 ): 8 m 2 /g [30]; peanut hull (FeCl 3 /N 2 ): 159 m 2 /g [31]; and lotus stem (FeCl 3 /O 2limited): 374 m 2 /g [32]. Increasing the FeCl 3 /biomass ratio could improve the activation process. However, an excessive ratio can afect the porous properties and application performance of MBC [23,33]. FeCl 3 should be well impregnated inside the natural holes of biomass. Terefore, high FeCl 3 loading may form bigger Fe-based particles and clusters, decreasing the surface area of catalytic Fe sites. Te bigger Fe-based particles may also block the pores of carbon bases, negatively afecting mass transfer. Te interaction between MBC and organic pollutants mainly comes from surface porous carbon with functional groups rather than Fe-based particles. High Fe loading content may decrease adsorption sites, causing weaker adsorption. To expand the porous system of MBC efectively, a few recent reports propose the combination of FeCl 3 with another activating agent during one-pot pyrolysis of biomass. By replacing N 2 with CO 2 , the S BET of MBC obtained from the FeCl 3 -activation of spent cofee grounds increased remarkably from 8 to 512 m 2 /g [30]. Hence, the additional activation during one-pot preparation of MBC is necessary, and the resulting material can be referred to as magnetic activated carbon (MAC).
Physical and chemical activation are the two most common techniques used to activate carbon-based materials [12]. Physical activation is a two-step technique that frst produces activated carbon by carbonizing biomass and then activates it at high temperatures with H 2 O or CO 2 [34,35].
For chemical activation, biomass resources are frst impregnated with activating agents such as KOH, K 2 CO 3 , H 3 PO 4 , H 2 SO 4 , AlCl 3 , ZnCl 2 , and FeCl 3 , and then carbonized and activated in one-step pyrolysis. Tose agents might theoretically activate MBC [36,37]. Te selection of an efective approach for expanding a porous system should not, however, impact the magnetic and other properties of the original MBC. Physical activation must be conducted within a range of high temperatures and high pressures, resulting in the potential for severe changes in the diferent properties of MAC products [34,35]. Terefore, chemical activation with a powerful activating agent like ZnCl 2 is preferable [38,39]. Experiments revealed that ZnCl 2 -activated carbon possessed a greater surface area and a greater number of micropores. In addition, the aromatic structure of ZnCl 2 -activated carbon was enhanced [40]. Hence, it is crucial to activate biomass with the combination of ZnCl 2 and FeCl 3 when few studies have been found in the literature. Lee and Ahmad Zaini [41] demonstrated that ZnCl 2 and FeCl 3 coactivation of palm kernel shell ofered MAC with a very high S BET of 1775 m 2 /g. As a result, the obtained MAC demonstrated exceptional adsorption of rhodamine B at 371 mg/g. Similarly, Lou et al. [42] prepared MAC from corn stover with a S BET of 1409 m 2 /g for signifcantly enhanced Cr (VI) removal of 185.8 mg/g. Based on the abovementioned results, it is possible to use both ZnCl 2 and FeCl 3 as an activating mixture in a facile one-pot preparation of MAC from biomass resources. In terms of adsorption, the superiority of MAC over MBC has been proven; however, its catalytic activity has been studied very little. Such reports [21,43] indicated that Fe-based particles in MBC could become catalytic sites for efective treatment of organic compounds through advanced oxidation processes. Tus, it is anticipated that MAC with Fe-based sites and an expanded porous system could exhibit better catalytic performance. To increase the possible use of MAC, its catalytic performance must be investigated in greater depth.
Nowadays, numerous industrial processes, including food processing, papermaking, printing, leather, textiles, cosmetics, and pharmaceuticals, discharge vast quantities of dyes into the aquatic environment [44][45][46]. Dye pollution is a signifcant environmental concern because synthetic dyes are typically not biodegradable, meaning they persist for extended periods of time in the environment [47,48]. In addition to causing aesthetic issues, dyes can impair the survival and reproduction of aquatic organisms [49][50][51]. Hence, efective remediation of dye pollution in wastewater is essential for environmental protection and sustainable development. Biodegradation, adsorption, coagulationfocculation, photocatalytic treatment, and chemical treatment are typical techniques [52][53][54][55]. With chemical treatment, dye molecules can be oxidized and broken down using chemicals, such as hydrogen peroxide, ozone, and persulfate, rendering them less toxic and simpler to remove from the environment [56]. Hydrogen peroxide (H 2 O 2 ) ofers several advantages over other oxidizing agents, including its safety, environmental friendliness, versatility, cost-efectiveness, simple operation, and mild conditions [57,58]. However, the use of H 2 O 2 alone is less efective. To accelerate the treatment of organic pollutants by H 2 O 2 , catalysts can be used. As mentioned before, magnetic biochar has proven that it is an efective catalyst for the treatment of synthetic dyes by H 2 O 2 owing to its efectiveness, stability, low cost, and environmental friendliness [27,59].
Lotus seedpods (LSP) are released from seed gathering in markets and factories, resulting in massive agricultural waste [60,61]. LSP is a prospective carbon resource for the production of various carbon-based products on account of its availability, abundance, underutilization, and low cost. In our previous studies, LSP was used to prepare MBC through one-pot FeCl 3 activation [19,27]. Te developed MBCs exhibited efcient catalytic activity for the elimination of organic contaminants by H 2 O 2 . Herein, LSP was continuously selected as a biomass resource for one-pot preparation of MAC using ZnCl 2 and FeCl 3 coactivation. To evaluate the catalytic performance of as-prepared MAC samples in a Fenton-like process, acid orange 10 (AO10), a synthetic azo dye with extensive usage, limited biodegradability, and potential toxicity [62][63][64] was selected.

Preparation of Magnetic Activated Carbon from Lotus
Seedpod. Magnetic activated carbon was prepared via the one-pot pyrolysis of ZnCl 2 and FeCl 3 -loaded lotus seedpod residue. First, 4.00 g of LSP powder, 0.80 g of FeCl 3 , and a certain amount (4x g) of ZnCl 2 were added to 100 mL of distilled water. After 3.0 h of stirring, the mixture was dried in an oven at 105°C for 24 h. Te dried sample was then added to a glass reaction tube in a vertical furnace. A constant nitrogen fow rate of 250 mL/min maintained the inert atmosphere inside the tube. To pyrolyze, the tube was heated from room temperature to 600°C at an average rate of 5°C/min and then held at that temperature for 60 min. Te obtained solid was washed repeatedly to remove all residual FeCl 3 and ZnCl 2 . Wastewater was tested with a pH meter, an electrical conductivity meter, an aqueous NaOH solution, and an aqueous AgNO 3 solution to detect ion leaching (Fe 3+ , Zn 2+ , and Cl − ). Lastly, the sample was dried at 80°C for 24 h to obtain MAC. Due to the mass ratio of ZnCl 2 /FeCl 3 /LSP being x/0.2/1.0, the as-prepared MAC samples were denoted as MAC-x. Moreover, biochar (BC), magnetic biochar (MBC), and activated carbon (ZAC), which served as reference samples, were prepared by the pyrolysis of LSP, FeCl 3 -loaded LSP, and ZnCl 2 -loaded LSP under the same procedure. Tese labels are presented in Table 1.

Characterization of Magnetic Activated Carbon.
Powder X-ray difraction (XRD) in the 2θ � 10-80°range was measured on a Bruker AXS D8 difractometer using CuK α radiation (λ �1.5418Å). Fe and Zn contents in MBC and MAC samples were analyzed by a Perkin Elmer Analyst 800 atomic absorption spectrophotometer (AAS). Tese metal elements were extracted from MBC and MAC samples in a HCl (6 M) solution at 60°C for 60 min. Nitrogen adsorption and desorption isotherms of MBC, ZAC, and MAC were measured at 77 K on a Micromeritics ® TriStar II Plus. All samples were degassed at 250°C for 5 h. Te specifc surface area (S BET ) was calculated from the Brunauer-Emmett-Teller equation. Te total pore volume (V total ) was determined at P/P o � 0.995. Te average pore size (d average ) was obtained from 4V total /S BET . Te pore size distribution was determined by the BJH method. Te magnetic properties of MBC and MAC were examined with a vibrating sample magnetometer (VSM) at room temperature. Fourier transform infrared (FTIR) spectroscopy of MAC was performed using a Tensor 27 spectrometer. Scanning electron microscope (SEM) images, energy dispersive X-ray (EDX) spectroscopy, and elemental mapping of MAC were analyzed using a JEOL JSM-IT200 instrument. Transmission electron microscopy (TEM) images of BC, MBC, and MAC were recorded by a JEOL JEM-1010 instrument.

Degradation of Acid Orange 10 Using Magnetic Activated
Carbon. Te catalytic performance of MAC samples was explored through the degradation of acid orange 10 using H 2 O 2 as an oxidizing agent at room temperature (30°C). In brief, 500 mL of AO10 (100 ppm) and a certain MAC dosage were added to a 1000 mL glass cylinder. Te initial pH value of the mixture was adjusted using H 2 SO 4 (0.5 M) and NaOH (0.1 M) solutions. Te adsorption step was carried out within the frst 20 min. Te adsorption capacity (Q), therefore, was calculated from the following equation: where C M (g/L) is the material dosage, and C A 0 and C A 20 (ppm) are the AO10 concentrations at the beginning and after 20 min of adsorption. After the adsorption step, the oxidation step was initiated by the rapid addition of H 2 O 2 to the mixture. Samples taken were added immediately to a solution of phosphate bufer and Na 2 S 2 O 3 (2.0 g/L) to adjust the pH to 7.0 and eliminate excess H 2 O 2 . AO10 concentrations were quantitatively examined at 480 nm with a UV-Vis spectrophotometer (Lovibond PC Spectro). Te decolorization efciency and total removal of AO10 were calculated as follows: Bioinorganic Chemistry and Applications 3 Decolorization where C O 0 and C O 30 (ppm) are the AO10 concentrations at the beginning and after 30 min of oxidation.
Te chemical oxidation demand (COD) was quantifed using the closed-refux titrimetric method (5220C) [65]. To minimize the infuence of residual H 2 O 2 on COD results, samples were mixed with a solution of 20.0 g/L Na 2 CO 3 and incubated at 90°C for 60 min [66].
To evaluate the stability and reusability of the MAC catalyst, a sample was used in fve consecutive experiments. Te used catalyst was recovered using a magnet, rinsed with distilled water and ethanol, and then placed in an oven at 110°C. Te dried catalyst was weighed in preparation for the subsequent experiment. At the end of each cycle, the treated solution was analyzed with the previously mentioned AAS instrument to identify Fe leaching.

Characterization of Magnetic
With ZnCl 2 activation, ZAC possessed the peaks at 2θ � 31. . Based on a report by Ma [40], the following equations might explain the production of ZnO: At high temperatures, molten ZnCl 2 can promote dehydration processes to cleave polymer chains of lignocellulosic biomass, yielding H 2 O and a thermoplastic carbonaceous phase [67]. ZnCl 2 can then combine with H 2 O to produce Zn 2 OCl 2 .2H 2 O. Subsequently, the decomposition of Zn 2 OCl 2 .2H 2 O can produce ZnCl 2 vapor, and its difusion can activate the thermoplastic phase to ofer the last porous carbon system [68]. Moreover, the formed ZnO can be kept in the carbon structure.
By  Together with ZnCl 2 , this reaction could activate the porous carbon system. In fact, as the ZnCl 2 /LSP mass ratio increased, the pyrolysis efciency fell marginally (Table 1). Despite the higher Zn-loading content, the stronger activation might reduce the remaining carbon content in MAC.

Porous Properties of BC, MBC, and MAC.
As presented in Table 1, S BET and V total of MBC were 274 m 2 /g and 0.14 cm 3 /g, respectively. Tese results are similar to previous studies for LSP-derived MBC [19,27]. With ZnCl 2 activation alone, S BET and V total of ZAC were 369 m 2 /g and 0.21 cm 3 /g, respectively. Te combination of FeCl 3 and ZnCl 2 was therefore expected to strongly enhance the porous properties of MAC. As a result, when the ZnCl 2 /LSP mass ratio increased from 0.1 to 0.4, S BET of MAC gradually rose from 531 to 1080 m 2 /g, which was 1.9-3.9 and 1.4-2.9 times more than that of MBC and ZAC, respectively. Similarly, V total of MAC samples was 0.31-0.51 cm 3 /g, which was 2.2-3.6 and 1.5-2.4 times higher than that of MBC and ZAC, respectively. Tese results demonstrate that the combination of ZnCl 2 and FeCl 3 improved the porous carbon system remarkably. Figure 2(a) displays the nitrogen adsorption and desorption isotherms for MBC and MAC-0.4. Extremely slim hysteresis loops resulting from capillary condensation indicated that few mesopores were formed. As a result, MBC and MAC were composed primarily of micropores. Indeed, BJH pore size distribution revealed that both the MBC and MAC-0.4 samples contained predominant micropores with similar typical pore sizes of around 1.2 nm (Figure 2(b)). Nonetheless, MAC-0.4 contained slightly more mesopores and macropores than MBC. Based on these fndings, coactivation led to a signifcant increase in the number of micropores and a moderate enlargement in pore size inside the carbon structure.

Magnetic Properties of MBC and MAC.
All MBC and MAC samples were easily attracted by an external magnetic feld from a magnet, as illustrated in Figure 3. Furthermore, VSM investigated their magnetic properties in depth. In general, all samples displayed similar magnetic hysteresis curves with extremely low coercivity, which was indicative of superparamagnetic behavior. Consequently, these materials may be magnetized and demagnetized simply. Similar trends have been uncovered in prior research [69,70]. In particular, MBC possessed a saturation magnetization of approximately 1.4 emu/g. Te saturation magnetizations of MAC-0.1, MAC-0.2, and MAC-0.4 were 1.9, 3.3, and 3.6 emu/g, which were 1.4, 2.4, and 2.6 times that of MBC, respectively. Tese results indicate that coactivation could enhance the magnetic properties of the obtained MAC. As presented in Table 1, the Fe content in all MAC samples (6.89-6.94 wt%) was not much higher than that in MBC (5.69 wt%). However, Fe 3 O 4 was predominant in MBC, whereas Fe 3 O 4 and Fe 0 coexisted in MAC samples. As previously discussed, when the ZnCl 2 / LSP mass ratio increased from 0.1 to 0.4, more Fe 0 crystals were formed. Consequently, the magnetic nature of different Fe-based materials may be the primary reason for the variation in the magnetic properties of MBC and MAC. According to Feng et al. [71], when Fe 3 O 4 was reduced to Fe, the magnetic properties of the resulting material increased because Fe can possess stronger magnetic properties than Fe 3 O 4 . In addition, other factors, such as the size, shape, magnetic anisotropy, and crystallinity of Fe 3 O 4 and Fe 0 , which strongly depend on the preparation conditions, could infuence their magnetic properties [72,73].  [74][75][76]. Notably, peaks at 525 cm −1 could be Fe-O bonds [77,78], and 466 cm −1 could be Zn-O bonds [41,79]. More importantly, the presence of polar oxygen-rich functional groups on the surface of MAC-0.4  Bioinorganic Chemistry and Applications could improve its interaction with organic pollutants and oxidizing agents during catalytic treatment processes.

SEM Images of MAC.
Te surface morphology of MAC-0.4 was observed by SEM images (Figure 5). Sharpedged fragments could be generated from vigorously crushing LSP. In addition, such macropores at the microscale were found. Depending on the porous properties, ZnCl 2 and FeCl 3 coactivation of LSP might afect micropores more than mesopores and macropores. Terefore, those macropores could come from the natural vascular bundles of LSP [80,81]. Especially, it seems that few Fe-and Zn-based particles were observed. Tese components may be embedded in the carbon framework without forming clusters on the MAC surface. Tis fnding is similar to that of MBC in previous studies [27,28]. Of particular importance, the frm immobilization is anticipated to enhance the stability and reusability of the MAC catalyst.

EDX Spectroscopy and Elemental Mapping of MAC.
EDX spectroscopy and elemental mapping were used to determine the chemical composition and elemental distribution on the surface of MAC-0.4 ( Figure 6). Te predominant elements included C (84.71 wt%), Fe (5.78 wt%), and O (8.19 wt%). Notably, the surface Fe content detected by EDX was close to the bulk Fe content analyzed by AAS (6.89 wt%). Te EDX result may show the surface distribution of Fe, whereas the AAS analysis may give the bulk Fe content (both outside and inside the carbon matrix). In traditional methods, the Fe element is normally decorated on the surface of the carbon base rather than inside the carbon framework. As a result, the surface Fe content from EDX may be much higher than the bulk Fe content from AAS. Herein, FeCl 3 was impregnated inside LSP. Hence, the distribution of Fe may be spread throughout the carbon  structure, resulting in comparable Fe contents from EDX and AAS results. Unlike Fe, the minor surface Zn content (0.29 wt%) was much lower than the bulk Zn content (3.94 wt%). It reveals that Zn on the carbon surface may readily be removed during pyrolysis. As previously indicated, ZnCl 2 vapor could be formed and difused into porous carbon. Due to its high mobility, ZnCl 2 vapor may escape of the MAC surface and be carried away by the fow of N 2 gas. Ten, only the inner carbon matrix may retain Zn better. Interestingly, the atomic ratio of O/Fe was approximately 5.0, which is much higher than that of Fe 3 O 4 . Tis comparison demonstrates that a considerable surface O content was present in the functional groups, as listed in the FTIR results.
For the remaining elements in MAC, Si and Cl were identifed at 0.45 and 0.59 wt%, respectively. Several reports demonstrate that minor elements, including Si, can be present in LSP [82,83]. However, Cl may be partially or entirely derived from the additional FeCl 3 and ZnCl 2 . As previously stated, MAC was cleaned until no Fe 3+ , Zn 2+ , or Cl − leaching was detected. Terefore, these elements could be frmly bound within the carbon matrix by strong mechanical or chemical linkages [27]. Lastly, element mapping showed that Fe, Zn, O, Cl, and Si elements were uniformly distributed on the carbon surface at the microscale. Te consistent spread of Fe and Zn may be a result of well-loaded FeCl 3 and ZnCl 2 in LSP. Following is a discussion on nanoscale TEM analysis for clarifying the interior structure of materials.

TEM Images of BC, MBC, and MAC. TEM images
were used to observe the internal structures of BC, MBC, and MAC-0.4 (Figure 7). BC shows a smooth surface with a gradual transition in brightness. Contrarily, the inconsistent brightness in MBC reveals the morphology of Fe 3 O 4 . At the nanoscale, dust-like Fe 3 O 4 particles were observed throughout the carbon matrix. Tese nanoparticles seem to group together in clusters. In addition to nanoparticles, MBC contained nanowires of Fe 3 O 4 . Intriguingly, the existence of magnetic nanowires in MBC is rare. It appears possible that Fe 3 O 4 nanowires may be formed in nanopores that resemble tubes [27]. Similar to MBC, MAC-0.4 had nanoparticles and nanowires that were well distributed throughout the carbon matrix. However, not only Fe 3 O 4 but also Fe 0 and ZnO crystals were present in MAC. It was suggested that the initial natural porous structure of LSP for ZnCl 2 and FeCl 3 loading played an important role in the morphology of Fe-and Zn-based products. LSP contains natural cellulose fbers [84]. Wirelike morphology may, therefore, result from crystallization in an extremely narrow fbrous matrix. More importantly, well-distributed Fe-based components at the nanoscale in the porous carbon system of MAC could not only improve its catalytic stability but also provide a greater contact area with other species for higher catalytic activity [78]. Tese advantages were explored in Fenton-like catalysis for the degradation of acid orange 10.  (Figures 8-13). Parameters, including MAC catalysts, MAC dosage, pH, and AO10 concentration, are presented in Table 2. All results revealed that the adsorption process closely reached equilibrium within 20 min before the next oxidation step. Although the experimental parameters were designed for catalytic oxidation, MAC exhibited excellent adsorption performance for AO10. In actuality, low MAC-0.4 dosages (0.10 to 0.40 g/L) eliminated AO10 with adsorption capacities ranging from 49.9 to 106.0 mg/g. Moreover, these quantities were much higher than those of BC, MBC, and ZAC. Tese results indicate that porous carbon systems (S BET and V total ) in carbon-based materials could play an important role in AO10 removal. Furthermore, π-π, hydrogen, and electrostatic interactions between functional groups on the MAC surface and AO10 [23,85] may aid in efective adsorption processes.

Efects of MAC Prepared by Diferent ZnCl 2 /LSP Mass
Ratios on AO10 Degradation. AO10 degradation was carried out with BC, ZAC, MBC, and MAC catalysts, as shown in Figure 8. BC removed a small amount of AO10, mainly by adsorption. With ZnCl 2 activation, ZAC eliminated 10.0% of AO10 through adsorption, and then almost lacked catalytic AO10 degradation in the subsequent step. Conversely, all MBC and MAC catalysts showed certain catalytic activity toward AO10 degradation. Tese results indicated that Febased components rather than ZnO and carbon-based support were the active sites for these catalytic processes. Te MBC sample contained Fe 3 O 4 crystals, while the MAC samples contained both Fe 3 O 4 and Fe 0 crystals. Tese Fe sites could catalyze AO10 degradation as follows (C− denotes that the Fe sites were incorporated into the carbon matrix) [77]: Compared with MBC, all MAC samples showed much faster AO10 decolorization rates. In addition, increasing the ZnCl 2 /LSP mass ratio improved the decolorization rate of AO10. As presented in Table 2, the Fe content in diferent MAC samples (6.89-6.94 wt%) was not much diferent and slightly higher than that in MBC (5.69 wt%). It reveals that other parameters, such as the nature, distribution, shape, and size of Fe-based crystals, may afect the catalytic activity of MBC and MAC. Several reports found that the composite of Fe 0 and Fe 3 O 4 exhibited higher catalytic performance than each component [86,87]. Te galvanic cell formed between Fe 0 and Fe 3 O 4 may facilitate electron transfer and •OH generation. MAC contained not only Fe 0 but also Fe 3 O 4 , which may follow this synergic efect. Furthermore, Fe-based components were fxed in the carbon supports, which could afect the process indirectly. MBC and MAC samples had diferent porous properties (S BET and V total ) and crystal structures of Fe-based components. As mentioned before, the nanoscale Fe sites were well dispersed in the porous carbon system of MAC-0.4, which had a high S BET and a large V total . Hence, mass transfer in these pores might become more convenient, and more catalytic sites with high residual energy might be accessible. Tese main advantages might explain the robust enhancement of the catalytic oxidation of AO10 by H 2 O 2 .
Te presence of minor elements in MAC might impact its catalytic activity. According to such reports, Cl − ions could be detrimental to AO10 degradation [88,89]. Te inhibitory efect of Cl − ions may be a result of their interaction with •OH. However, MAC was carefully rinsed to remove all water-soluble components. Consequently, trace quantities of Cl and Si may not exist as ions or be frmly bound within the carbon framework. It may be challenging to leach those elements into the treatment media. Due to the strong catalytic activity of MAC on AO10 degradation, the signifcance of these trace elements may be negligible. Figure 9 depicts the relationship between MAC dosage and AO10 degradation. Without a catalyst, it was nearly impossible for H 2 O 2 to eliminate AO10. In contrast, when MAC was applied, AO10 decolorization occurred rapidly. With MAC dosages between 0.20 and 0.60 g/L, AO10 was nearly completely decolored within 30 min. Tese results demonstrated that MAC catalyzed this decolorization effectively. In addition, the decolorization rate generally increased when the MAC dosage rose from 0.10 to 0.   literature, the excess catalyst might deactivate the originally generated •OH radicals, as shown in the following equation [90][91]:

Efects of pH on AO10 Degradation Catalyzed by MAC.
pH can be a crucial variable for AO10 degradation catalyzed by MAC on the basis of the Fenton-like mechanism. As shown in Figure 10, AO10 degradation in the pH range of 2.0-5.0 was investigated.  Figure 11). As presented in Table 3, the average decolorization rates in 30 min at 100 and 200 ppm AO10 were 16.7 and 33.3 mg AO10/g MAC/min, respectively. Tese results demonstrated that MAC could catalyze AO10 degradation efectively over a wide concentration range. Te Fenton-like catalytic performance of MAC for AO10 degradation was compared with that of other catalysts (Table 3). In general, most catalysts require long treatment times and high catalyst dosages at low AO10 concentrations. In a previous study, LSP-derived MBC showed good catalytic activity for AO10 degradation, with almost all AO10 being decolorized within 90 min [27]. Its average decolorization rate was 2.8 mg AO10/g MBC/min, which was much higher than that of other catalysts. However, at a similar condition, the average decolorization rate catalyzed by MAC was 6.0-fold higher than that by MBC. Tese comparisons prove that the catalytic performance of MAC is superior to that of other catalysts. As discussed before, well-   3.2.6. COD Reduction during AO10 Degradation Catalyzed by MAC. COD is defned as the total amount of oxygen required for the oxidation of organic matter into CO 2 and H 2 O [97]. It is an important parameter to determine the degree of mineralization during the treatment of organic compounds and is subject to strict regulation by environmental regulatory agencies [98]. Here, changes in COD and AO10 concentrations during Fenton-like degradation catalyzed by MAC were carried out (Figure 12). At the beginning, 100 ppm AO10 provided 91 mg/L COD. In the initial adsorption step, AO10 and COD concentrations were lowered in part. With the MAC-catalyzed acceleration, the AO10 concentration in the subsequent oxidation process fell rapidly and nearly complete in 30 min. At the same time, COD declined gradually from 88 to 53 mg/L, and this tendency continued throughout the later period. At 120 min, COD dropped to 31 mg/L, corresponding to 66% of COD elimination. Tus, despite the fact that AO10 was decolorized during the frst period, certain organic intermediates might require additional time to be completely mineralized [99,100]. Unselectively, reactive •OH radicals can attack species. As a result, AO10 can be converted into numerous intermediates like aniline, phenol, 7-hydroxy-8-(hydroxyamino) naphthalene-1,3-disulfonic acid, 7,8-dihydroxy-naphthalene-1,3-disulfonic acid, alpha naphthol, and carboxylic acid. To completely mineralize AO10, additional    treatment time may be necessary, or the Fenton-like process can be combined with other treatments [27,96].

Stability and Reusability of MAC Catalyst.
Catalyst stability and reusability play crucial roles in industrial pollutant remediation. In order to investigate those characteristics of the MAC catalyst, fve consecutive cycles of AO10 degradation were performed in triplicate. Figure 13 depicts the mean values for the experiments. After 20 min of adsorption, H 2 O 2 was added to each cycle, and the treated solution was analyzed for Fe leaching. As a result, the catalytic performance of MAC-0.4 remained efective even after fve cycles. At 20 min of oxidation, the total AO10 removal after each cycle was 93.9 ± 0.9%, 88.4 ± 3.5%, 86.9 ± 0.9%, 86.2 ± 1.4%, and 86.3 ± 0.8%, respectively. Te removal decreased slightly in the second cycle, then stabilized in the subsequent three cycles. It appears that unstable Fe sites were leaked into the treated medium in the frst step. Te remaining Fe sites in a recycled catalyst may be frmly anchored in the MAC framework and ofer stable catalytic performance in the following cycles. Furthermore, the adsorption capacity of MAC on AO10 decreased with each cycle. It seems that distilled water and ethanol cannot eliminate adsorbates entirely. Consequently, it may impact the catalytic performance of the used MAC-0.4 in the subsequent cycle. Lastly, AAS results revealed that 1.14-1.19 mg/L of Fe leaching was detected after each cycle. Tis leaching was below the limit concentration of 2 mg/L established by European Union directives for treated water.

Conclusion
In summary, magnetic activated carbon was successfully prepared using one-pot pyrolysis of ZnCl 2 and FeCl 3 -loaded lotus seedpod waste. Te as-prepared MAC had a high S BET of 1080 m 2 /g, a large V total of 0.51 cm 3 /g, and a strong saturation magnetization of 3.6 emu/g, which were 3.9-fold, 3.6-fold, and 1.8-fold higher than those of MBC. With 6.89 wt% Fe and 3.94 wt% Zn, diferent crystals of Fe 3 O 4 , Fe 0 , and ZnO were present in MAC. Interestingly, TEM images showed that their nanoparticles and nanowires were developed inside the carbon matrix. Subsequently, MAC was investigated for the treatment of acid orange 10. As a result, MAC demonstrated both a useful adsorbent and an efcient Fenton-like catalyst. At pH 3.0, 0.20 g/L MAC removed AO10 (100 ppm) with an adsorption capacity of 78.4 mg/g. When 350 ppm of H 2 O 2 was added, AO10 decolorization occurred rapidly and was practically complete within 30 min. At 120 min, 66% of the COD was removed. Moreover, the catalytic performance remained stable, with total AO10 removal slightly decreasing from 93.9 ± 0.9% to 86.3 ± 0.8% after fve consecutive cycles. Te minimal iron leaching ranged from 1.14 to 1.19 mg/L. In conclusion, these results indicated that magnetic activated carbon derived from ZnCl 2 and FeCl 3 coactivation of lotus seedpod residue is an efcient catalyst for robust acid orange 10 decolorization.

Data Availability
No new data were created or analyzed in this study.

Conflicts of Interest
Te authors declare that they have no conficts of interest.